Flat-panel display

ABSTRACT

A video display for two or three dimensions has a flat liquid-crystal screen  2  which ejects light from the plane at a selectable line  3 . One or, in the case of a 3-D display, several video projectors  1  project a linear image into the plane from an edge. A complete image is written on the screen by addressing the line with appropriate images as it is scanned down the screen. To screen a three-dimensional image the video projectors, each projecting an image as seen at a slightly different angle, combine to constitute a three-dimensional display which produces a three-dimensional image that is one line high.

This invention relates to displays, particularly to the video display ofthree-dimensional images.

Video displays commonly present moving two-dimensional images for use intelevision and as computer monitors. It is becoming possible to displaythree-dimensional images on such displays, and a variety of designs havebeen proposed.

It is well known that holograms are capable of displayingthree-dimensional images. A crude hologram can be written by ahigh-resolution liquid-crystal display, so such displays are capable ofdisplaying video three-dimensional images. But it is difficult to makeliquid-crystal displays with pixels less than five microns wide, andholograms produced on such displays have a narrow field of view.

Just as video two-dimensional images can be displayed by raster-scanninga two-dimensional screen, a three-dimensional image can be displayed byraster-scanning a three-dimensional volume. One way of doing this is toscan a volume of a suitable material with a pair of laser beams. If thematerial is transparent to both laser beams, and if the material emitslight at any point where the laser beams intersect, then it is possibleto cause the point of intersection of the laser beams to sweep out thewhole volume so as to write the three-dimensional image. The problemwith displays like this is that the images which they produce arenecessarily transparent, i.e. there is no provision for images at thefront of the volume to block light from images at the rear.

More common is the technique whereby a conventional video displayscreens left and right views alternately of a three-dimensional object,and the viewer wears a pair of spectacles which make one of the pairvisible to each eye. If this is done quickly enough for flicker to beimperceptible, the viewer sees a stereo image which allows theperception of depth.

Spectacles however get lost. An alternative system is to place an arrayof lenslets over a two-dimensional display and provide two pixels undereach lenslet. Provided the viewer is positioned correctly it can bearranged that one pixel under each lenslet is imaged to the viewer'sright eye and the other to the viewer's left eye. The viewer then sees astereo image without having to wear spectacles.

The difficulty with screening only two views in this way is that theviewer must keep his head in the correct position. If more than twopixels are provided under each lenslet it becomes possible to screenseveral views of the three-dimensional image. This has the dualadvantage that the viewer can see a stereo image over a wider range ofpositions, and that the viewer can inspect the three-dimensional imagefrom a variety of angles.

For the lenslet system to work, the two-dimensional display must have ahigh resolution and this makes construction of the display difficult. Adifferent approach is to adapt a rear-projection two-dimensional displaywhich commonly comprises a video projector and translucent screen. Ifthe translucent screen in such a display is exchanged for a lens thenlight from the video projector will as before form a video image whichwill lie in the plane of the lens, but after passing through the lensthe light will converge to a zone at some point in space which in factis an image of the video projector. If the viewer puts one eye in thiszone they will be able to see a two-dimensional image but only with thateye. Put another video projector adjacent to the first and a view can bedisplayed to the viewer's other eye, and indeed further projectors canbe added to display more views to adjacent positions into which theviewer's head might stray. Such a system is shown in FIG. 1. This systemcan be assembled from projectors of conventional resolution, but theprojectors must be positioned with great precision, and the projectorlenses must be designed so that the aperture stop of each lens isdirectly adjacent to those of neighbouring lenses.

An alternative approach is to use a single video projector and lens, butto place a multi-element shutter in front of the video projector lens,as shown in FIG. 2. As before, light from the video projector forms avideo image on the lens and then is focused by the lens to a zone, butthe zone now comprises an image of the video projector with the shutterin front. By choosing a lens with a different focal length the zone canbe expanded until each element of the shutter is imaged to a sizesuitable for one eye so that if only one shutter is open and all othersare closed, the image on the lens is visible to a single eye. Views of athree-dimensional image can be screened one by one on the videoprojector and a different element of the shutter opened and closed foreach view in such a way that provided the sequence is repeated quickly,the viewer sees a three-dimensional image. The problem with this systemis that it wastes light and is bulky.

Yet another approach (FIG. 3) is to use a liquid-crystal display whichis illuminated in such a way that it is visible from only one positionat any time. Views of the three-dimensional image can be screened one byone on the liquid-crystal display, and the direction of illuminationswitched as each view is screened in such a way that each view isvisible to a different area. Provided this is repeated quickly enoughthe flicker of each view need not be perceptible, and the viewer sees athree-dimensional image much as with the lenslet system. A display ofthis type is shown in the inventor's earlier patent GB-B-2206763.Switching the illumination of a liquid-crystal display in this way meansthat the optics need not be precisely registered, and the liquid-crystaldisplay need not have a high resolution. A switched illumination schemerequires a liquid-crystal display with a high frame rate and suchdisplays have been demonstrated. But like most modern liquid-crystaldisplays a matrix of transistors is needed to make a high frame ratedisplay, and it is expensive to manufacture even small versions of suchdisplays.

Liquid-crystal displays are not the only flat-panel displays which areexpensive to manufacture. Most flat-panel displays comprise a matrix ofindividually controlled pixels and since there may be almost a millionof these great care is needed to ensure that none fails. The screen of acathode ray tube by contrast is a uniform layer of material which,because it has no detail or structure, rarely fails and therefore makesthe cathode ray tube rather simple to construct. It is because thepicture is built up by raster-scanning that the screen of a cathode raytube can be so unstructured, and it appears to have been assumed thatone cannot raster-scan a flat-panel display because the display needs acertain depth for the scanning beam.

However if a cathode ray tube is designed to produce a video imagecomposed of a single line of pixels it can be made essentially flat.Indeed any video system which is designed to produce a single line ofpixels can be made flat. With such flat systems all the opticalcomponents can be usually be confined within the core of a slabwaveguide, and even if the system is long or wide it can be packedbehind a flat screen simply by using mirrors to fold the optical layout.All that is needed is some device at the end of the optical system toexpand the height of the display in order to restore the lost dimension.

Of particular interest here are video projectors and three-dimensionaldisplays. A video projector is a device which projects light thatfocuses to form a video image at some point in space, and a one-linevideo projector is for the purposes of this document defined to be avideo projector which writes a video image comprising a single line ofpixels. Similarly a three-dimensional display is a device capable ofdisplaying a video three-dimensional image, and a one-linethree-dimensional display is for the purposes of this document definedto be a three-dimensional display which produces an image that is asingle line high.

According to the present invention there is provided a flat-paneldisplay comprising means for modulating the intensity and direction of aray, a flat layer or panel which is not opaque to the ray, themodulating means being arranged to direct the ray into a side of thepanel and the panel being adapted to enable the emission of light at thepoint where the ray intersects a selected zone of the panel, and meansfor selecting the zone.

The invention can be used to make a 2-D display which is simply scannedline by line in much the same way as, say, a CRT screen, each lineconstituting the respective zone and being produced by a correspondingmodulation of the preferably single-line modulating means. Here theselected zone need only scatter the light.

Preferably however the invention is used to make a 3-D display where notonly the position on the plane at which light is emitted is significant,but also its direction. To this end the modulating means can consist ofseveral individual modulators, each capable of generating atwo-dimensional image, for instance by multiplexing of single-linecomponents as previously, and each injecting this image into the panelat a slightly different angle. If the emission from the panel is at anangle corresponding in a suitable way to the angle of entry then thedifferent views projected can be made to correspond to views from theangles in question and a 3-D effect is obtained.

For a better understanding of the invention embodiments of it will nowbe described by way of example with reference to the accompanyingdrawings, in which:

FIG. 1 shows a previous way of making a three-dimensional display from alens and several video projectors;

FIG. 2 shows a previous way of making a three-dimensional display from alens, a high frame-rate video projector, and a liquid-crystal shutter;

FIG. 3 shows a previous way of making a three-dimensional display from alens, a high frame-rate liquid-crystal display and a scanning spotsource of light;

FIG. 4 shows a flat-panel display representing a first embodiment of theinvention, which screens a two-dimensional image;

FIG. 5 shows a close-up of the liquid-crystal panel used in theflat-panel display of FIG. 4;

FIG. 6 shows a flat-panel display which screens a three-dimensionalimage, as a second embodiment;

FIG. 7 shows the construction of the several one-line video projectorsused in the flat-panel display of FIG. 6;

FIG. 8 shows a way of creating a variable optical path length;

FIG. 9 shows a third embodiment being a flat-panel display with avariable optical path length which screens a three-dimensional image;

FIG. 10 shows another way of creating a variable optical path length;

FIG. 11 shows a fourth embodiment in the form of a flat-panel displaywhere a two-dimensional image is created by scanning the intersection oftwo rays; and

FIGS. 12 and 13 show fifth and sixth embodiments, namely flat-paneldisplays where a two-dimensional image is created by scanning atravelling wave with a one-line video projector.

Referring to FIG. 4, the display comprises a one-line video projector 1including a display panel 18 and a projection lens 20, a panel 2,capable of displaying a multiplicity of lines input from the videoprojector one at a time over its area, and means 3 for selecting one ofthe multiplicity of parallel lines on the panel. The panel 2 may be aliquid-crystal panel addressable in lines 3 perpendicular to theoncoming beam.

A picture is written line by line onto the panel by selecting one lineat a time of the liquid-crystal panel 2 and illuminating the line withthe appropriate pattern of light from the one-dimensional videoprojector 1.

The liquid-crystal panel 2, as shown in FIG. 5, is made of aliquid-crystal layer 4 between upper and lower glass plates 5. It actsnot as a display but as a slab waveguide with liquid crystal 4 on oneside of the lower glass slab 5 b and air on the other. Light from theone-dimensional video projector 1 is shone into one edge of theliquid-crystal panel 2, namely into the lower slab 5 b, and is emittedfrom its upper face when the light intersects whichever one of the lineson the panel has been selected by the means 3 for line selection.

The liquid-crystal here is ferro-electric, for example, and issandwiched in the conventional way between the two slabs of glass 5. Theangle at which light L is injected into the rear slab 5 b and therefractive indices of it and the liquid crystal 4 are chosen in such away that in one state the liquid crystal 4 reflects light by totalinternal reflection and in the other state a substantial proportion ofthe light escapes through the upper slab 5 a to be directed towards theviewer.

At the interfaces between liquid crystal 4 and slab 5 there is the usualalignment layer 6 and layer of transparent conductor 7. The latter inthis instance is patterned and electronically controlled so that laterallines of the liquid crystal 4, perpendicular to the incoming ray, can beswitched. These lines would normally be horizontal as presented to aviewer. Furthermore reflector means 8 must be provided so that light ispermanently reflected from spaces between adjacent lines of the lowertransparent conductor 7. The upper conductor 7 a is here continuous,though it would be possible to reduce the number of address lines neededby a multiplexing scheme in which the upper conductor was also dividedinto areas each addressing a group of lower electrode lines.

In use the LC panel 2 is addressed line by line, while for each line aone-dimensional series of points corresponding to the desired picture isproduced by the projector 1. The light from the projector, thusmodulated, passes through the slab until it reaches the row currentlyaddressed, whereupon it escapes, as shown by the broad arrow in FIG. 4.This appears to the viewer as a modulated line. By scanning the surfaceof the plane of the liquid-crystal panel a 2-dimensional image is builtup, at the end of which the next frame starts.

An undesirable aspect of this display is that the one-line videoprojector 1 must be far from the screen if there is not to besignificant key-stone distortion of the image. This can be resolved byplacing a lens 9 adjacent to the liquid-crystal panel and putting thevideo projector 1 approximately in the focal plane of the lens 9(compare FIG. 6). A further feature of the display of FIG. 4 is that thepicture has a narrow field of view, because light escapes only at oneangle. However, the field of view can be increased simply by placing atranslucent screen 10 between the viewer and the panel 2 and adjacent tothe panel 2.

The narrow field of view can instead be conveniently turned to ouradvantage so as to produce a three-dimensional display. Furtherembodiments of the invention will therefore be described in which thereis provided a flat-panel display capable of screening athree-dimensional image. In FIGS. 6 and 7 the display comprises aone-line three-dimensional display 11, a liquid-crystal panel 2 andmeans 3 for selecting a line on the liquid-crystal panel. In general,similar parts are given the same reference numerals.

The one-line three-dimensional display or source 11, partially shown inFIG. 6, comprises a lens 9 and several one-line video projectors 1, hereshown magnified, approximately in the focal plane of the lens 9. Thenumber of projectors depends on the application; ideally for, say, a 1°resolution in angle and an adequately wide field of view there should beeighty projectors, all in a line. As shown in FIG. 7, the array ofone-line video projectors 1 is formed by an assembly consisting of red,green and blue lasers 12, a colour combiner 13, a cylindrical expansionlens 14, an array of cylindrical lenslets 15, a colour splitter 16, acylindrical condenser lens 17, one liquid-crystal display 18 for eachprojector 1, a colour combiner 19 and one generally rectangularcylindrical projection lens 20 for each projector 1. The one-line videoprojectors 1 are arranged so that each edge of the aperture of eachcylindrical projection lens 20 coincides with an edge of the aperture ofthe adjacent lens 20 a. FIG. 6 also shows a one-dimensional prism 31used for directing the light into the slab at the correctnear-TIR-maximum angle.

In use, light from the lasers 12 is combined into a single beam by thecolour combiner 13 and then expanded in the plane of the display by thecylindrical expansion lens 14. The array of cylindrical lenslets 15behaves like a screen which is translucent in the plane of the displaybut transparent orthogonal to the plane, so that light is scatteredwithin the plane. The light is then split by the colour splitter 16 intothe three separate colours and passes through the cylindrical condenserlens 17 onto each liquid-crystal display 18, where it is modulated. Thedisplay 18 contains mirror surfaces which reflect the light back throughthe cylindrical condenser lens 17 which condenses the three colours tothe colour combiner 19, where they are combined into the cylindricalprojection lenses 20.

Each LC display element 18 is made up of a number of pixels in the formof columns, the number of columns corresponding to the number of pixelsacross the plane of the display. The display on each display element 18corresponds to a view from a particular angle, or to a one-line sampleof that view. The corresponding lens 20 spreads the reflected light fromthe display elements 18 towards the collimating lens 9, which directsthe beam into the panel at an angle to the axis of the system. Therelevant line of the 2-D wave-guide 2 that is activated at any giventime then extracts the input from each LC element 18 at an anglecorresponding to its incoming angle. Hence as the panel 2 is observed adifferent view is seen depending on the angle across the panel, i.e. inthe plane of the panel, perpendicular to the axis of the system (bottomleft to top right in FIG. 6).

Normally 3-D displays require the 3-D effect only across the view, notup and down. A diffuser screen 10 is therefore applied which is like thescreen in FIG. 4 in that it scatters or spreads light up and down (inelevation), so that the images can be viewed at whatever height theviewer is located, but unlike that screen conserves angle in azimuth(across the panel), to achieve the 3-D effect. The drawing of the screen10 schematically shows corresponding cylindrical lenslets to achievethis effect, though in practice the lenslet side would be the underneathside, adjacent to the panel 2.

The number of wire connectors in this system will be large if a wireconnection is made to every pixel of the one-line video projectors 1.Instead the liquid-crystal display 18 in each one-line video projector 1comprises a two-dimensional liquid-crystal array with a back-plane ofdigital silicon transistors that can demultiplex digital signals from afew wire connections to the many liquid-crystal pixels. Each column ofthe liquid-crystal display 18 is conceptually split into three—onecolumn for each colour—and grey scale is achieved by altering the sizeof the transparent area within each column.

In a conventional three-dimensional display which comprises a lens andvideo projectors it is difficult to make either edge of the aperture ofeach projection lens coincide with the aperture of the adjacent lens.This is because if the quality of the three-dimensional image is to bereasonable the lenses must also have a low F-number and wide field ofview. To make the lenses 20 for this embodiment a layer of photoresistis placed on a slab of low-refractive-index glass. The photoresist ispatterned, developed and etched and the exposed areas are filled withmaterial of a different refractive index. A second slab of lowrefractive index glass is placed on top and the result is a slabwaveguide whose core contains optical elements. The optical elements maybe a series of aspheric profiles, or a two-dimensional hologram, eitherof which has the ability to provide good imaging properties at low cost.

A problem with the three-dimensional display just described is that theimage from each video projector 1 has a single focal line, which cancoincide exactly with only one line on the liquid-crystal panel 2. Thiswill lead to image anomalies and it is better if the video projectors 1refocus on each line of the panel 2 as it becomes transparent.

A further embodiment of the invention with this in mind is thereforeshown in FIGS. 8 and 9. Here there is provided a flat-panel displaycapable of screening a three-dimensional image free of the anomaliesdescribed. The display comprises a one-line three-dimensional display11, a medium 21 of variable optical path length, a first one-dimensionalretroreflector 22, a liquid-crystal panel 2 and means 3 for selecting aline on the liquid-crystal panel 2.

The medium of variable optical path length 21 comprises a polarisingbeam splitter 23, a quarter-wave plate 24, a second liquid-crystal panel25 underneath the first, means 26 for selecting a line on the secondliquid-crystal panel and a second one-dimensional retroreflector 27.

In use, light from the lasers 12 passes through elements 13, 14, 15, 16and 17 and is reflected by the LCDs 18 through element 19. The lightthen passes through elements 20 and 9, and into the beamsplitter 23.Light is transmitted by the polarising beam splitter 23 and through thequarter-wave plate 24 into one edge of the liquid-crystal panel 25. Oneglass slab of the liquid-crystal panel 25 is configured to act as awave-guide, and light is freed from the waveguide at whichever line isselected. The one-dimensional retroreflector 27 is positioned so thatlight is retroreflected back along its path into the guiding slab of theliquid-crystal panel 25 as far as its forwards or axial direction isconcerned, while the component of the light in the direction that isorthogonal to the plane of retroreflection is simply reflected. Thelight then passes back through the quarter wave plate 24 and, itspolarisation by this stage reversed, is reflected by the polarisingbeam-splitter 23 onto the one-dimensional retroreflector 22 and via afurther prism into the slab.

The one-dimensional retroreflector 22 is likewise configured to reflectthe light into the liquid-crystal panel 2, but to retroreflect thecomponent of light orthogonal to the direction of reflection. Theretroreflector acts like a lens which provides unit magnificationimaging regardless of optical path length. If the length of the variableoptical path 21 is made, by appropriate addressing of the second panel25, always to equal the length from the first retroreflector 22 to theselected line, then it can be arranged that the video projectors 1always come to focus on the selected line.

The variable optical path length 21 may alternatively comprise severalwaveguides of differing lengths and means for re-routing light throughvarious of these waveguides. FIG. 10 shows two such waveguides 28 withthe light routed through the shorter of the two, in the manner of anoptical trumpet. The light is then reflected off the one-dimensionalretroreflector 22 and passes into the main body of the liquid-crystalpanel 2 from which it is emitted at the selected line. The advantage ofthis alternative is that the same liquid-crystal panel 2 can be used fordisplaying the image and for providing a variable optical path length21.

A further embodiment of the display, as shown in FIG. 11, comprises alaser and one-dimensional scanner 1, a second laser and one-dimensionalscanner 3 and a flat layer of material 2, such as Er³⁺-doped CaF₂, whichemits light at the point where the two laser beams intersect inproportion to the intensity of the laser beams. The two-dimensionalpicture is written pixel by pixel onto the flat layer of material byraster scanning the point where the laser beams intersect and modulatingthe lasers.

A further embodiment of the display, FIG. 12, comprises a cathode raytube and projection lens 1, a sheet of reflective foil 2, and atransducer which can create solitary horizontal surface waves in thefoil 3. The transducer creates a single wave and the projection lensimages the cathode ray tube onto the wave. The front of the wavereflects light from the cathode ray tube into the viewer's eyes so thatthe viewer sees an image of the cathode ray tube line at the position ofthe wave. As the wave travels down the screen the cathode ray tubewrites further lines of the picture until the whole is complete, then anew wave is set up by the transducer ready to write the next frame.

FIG. 13 shows a yet further embodiment in which the slab waveguide 2 isin the form of a transparent sheet folded over on itself to produce atall inverted U-section. Instead of light being reflected off thesurface of the slab it is fed in at one end of the U, remaining in theslab by TIR, and is extracted from the slab by the presence of alocalised surface wave. Thus light from an arc lamp (or some other pointsource) 31 is focused by a cylindrical lens 35 into a slab waveguide 2.Meanwhile it is modulated in azimuth as it spreads out by a spatiallight modulator 33. This produces a set of diverging rays in the planeof the waveguide, each ray at the intensity of a pixel in the currentline. The rays are collimated by the optical lens 37, go round the curve2 a, then are scattered by the surface wave into the viewer's eyes.

The surface wave 41 is set up by the surface wave transducer 43 andpropagates up the slab wave-guide until it is absorbed by the surfacewave damper 45. Meanwhile pixels are addressed onto the surface wave bythe optical system so that a picture is built up line by line. The wavecould be of an S-shape instead of a simple bump in cross-section.

Further embodiments and variations are possible as follows.

Examples of a one-dimensional video projector 1 are a lens with thefollowing approximately in the focal plane; a one-line cathode ray tube,a one-line spatial light modulator such as a line of deformable mirrorsand a screen scanned by a laser. The ray can be a ray of light, a soundwave or a surface wave.

Examples of means for modulating the intensity and direction of a rayare as follows: a modulator and near-field scanner, a modulator andfar-field spatial light modulator, a beam expander, and spatial lightmodulator and projection lens.

Examples of a flat layer of material which is not opaque to the ray andenables the emission of light at the point where the ray intersects aselected zone of the material are as follows: a flexible mirror with asurface wave which reflects light, a flexible slab waveguide with asurface wave whose radius of curvature causes light to be emitted, aslab waveguide with a photoelastic core such as perspex or urethanerubber in which an acoustic wave causes the emission of light, atwo-photon absorption material which absorbs light at the point ofintersection of two laser beams and thereupon emits light, a slabwaveguide with a liquid-crystal cladding layer which reflects lightalong the guide in one state and transmits it in the other, a sheet ofcylindrical lenslets spinning about an axis in the plane of the sheet inparallel with the axis of the lenslets.

Examples of a one-line three-dimensional display 11 are a one-lineliquid-crystal hologram, a one-line acousto-optic hologram, a one-linehigh volume display, a one-line display and lenslet array, a one-lineliquid-crystal display with lens and scanning spot source of light inthe focal plane of the lens. If holograms are used then some of thelenses in the embodiments shown become redundant.

Examples of a variable optical path length include a movable mirror(i.e. the optical equivalent of a trombone) and a device which switcheslight between different lengths of waveguide (i.e. the opticalequivalent of a trumpet).

What is claimed is:
 1. A display of the flat-panel type, comprising asource (1) of one or more rays, means for modulating the intensity ofthe rays, a panel (2) which is not opaque to the rays and enables theemission of light at some position along each ray when the ray isdirected into a side of the panel, and means (3) for selecting suchpositions; wherein the display includes a projector (1), containing themeans for modulating the intensity of the rays, for modulating the inputrays line by line for input into the panel, and the selecting meanscorrespondingly selects one line at a time on the panel so as to displaythat line from the projector.
 2. A flat-panel display as in claim 1,wherein the rays are of visible light, and the emission of light fromthe panel (2) is brought about by deflection of the rays at the selectedposition.
 3. A flat-panel display as in claim 2, wherein the panelincludes a reflective sheet and a transducer (43) for producing alocalised linear acoustic or surface wave in the sheet (2), the presenceof the wave at a given position causing reflection of the ray and thusthe said deflection at that position.
 4. A flat-panel display as inclaim 3, wherein the means for selecting the position of deflection isadapted to cause the transducer (43) to excite the acoustic or surfacewave and to wait until it reaches the required position before the rayis emitted.
 5. A flat-panel display as in claim 2, wherein on one sideof the panel (2) is a layer (4) which is switchably reflective ortransparent, and the means for selecting the position at which thedeflected rays are emitted from the panel is adapted to change the stateof the switchable layer at a time.
 6. A flat-panel display as in claim1, wherein the projector means is adapted to produce a one-linethree-dimensional display and to this end comprises a set of individualone-line projectors (1), each of which is adapted to display a line ofthe image seen at a different (azimuthal) angle, the rays from each ofthese one-line projectors being emitted from the panel at thecorresponding azimuthal angle.
 7. A flat-panel display as claim 1, inwhich the or each projector is formed as a row of addressable pixelsparallel to the said edge of the panel.
 8. A display of the flat-paneltype, comprising a source (1) of one or more rays, means for modulatingthe intensity of the rays, a panel (2) which is not opaque to the raysand enables the emission of light at some position along each ray whenthe ray is directed into a side of the panel, and means (3) forselecting such positions, wherein the display is adapted to produce athree-dimensional image and to this end comprises a ray-projectingdevice (1), for directing the modulated rays into the panel at variousazimuthal angles, the rays being emitted from the panel as they areselected by the selecting means (3) at respective azimuthal anglescorresponding to their angle of entry into the panel.
 9. A flat-paneldisplay as in claim 8, further including means for applying a variableoptical path length, the optical path including a retro-reflector, insuch a way that the total optical path length between theretro-reflector and the respective line on the panel, minus the opticalpath length between the retroreflector and the ray source, remainsapproximately constant.
 10. A flat-panel display as in claim 9, whereinthe variable-length optical path includes a transparent layer (25) fortotal internal reflection, on one side of which is a layer that isswitchably reflective or transparent.
 11. A display of the flat-paneltype, comprising a source (1) of one or more rays, means for modulatingthe intensity of the rays, a panel (2) which is not opaque to the raysand enables the emission of light at some position along each ray whenthe ray is directed into a side of the panel, and means (3) forselecting such positions, wherein the panel includes a reflective sheetand a transducer (43) for producing a localised linear acoustic orsurface wave in the sheet (2), the presence of the wave at a givenposition causing reflection of the ray and thus the said selection ofthat position.
 12. A method of displaying an image, in which lightmaking up successive lines of the image is directed into the side of apanel containing a flat layer (2) of modulatable material, and thematerial is addressed a line at a time to modulate it and thus to causeemission over the plane of the layer so as to build up the image.